The present invention relates generally to methods of manufacturing photovoltaic silicon solar cells, and more specifically to back-contacted solar cells fabricated using a gradient-driven migration process, such as thermomigration or electromigration, to create an array of n++ conductive vias electrically connecting an n+ emitter surface on the front side of the cell to a grid of ohmic contacts located on the cell's backside.
Back-contacted silicon solar cells (photovoltaic cells) have several advantages compared to traditional silicon solar cells that have ohmic contacts located on both the front and rear surfaces. One advantage is that back-contacted cells have a higher conversion efficiency due to reduced (or eliminated) contact-obscuration losses (e.g., sunlight reflected from a front-side contact grid is unavailable to be converted into electricity). Another advantage is that assembly of back-contacted cells into electrical circuits will be easier, and therefore cheaper, because both polarity contacts are on the same surface (i.e., the back-side). As an example, significant cost savings compared to present photovoltaic module assembly can be achieved with back-contacted cells by encapsulating the photovoltaic module and assembling the solar cells into an electrical circuit in a single step. Another advantage of a back-contacted cell is better aesthetics by providing a more uniform appearance (e.g., an all-black surface). Aesthetics is important for some applications, like building-integrated photovoltaic systems (e.g., solar roof tiles), and photovoltaic sunroofs for automobiles.
An important issue for back-contacted silicon solar cells is identifying a cell design and fabrication process that are inexpensive to manufacture and that can use inexpensive, lower-quality silicon substrates. Solar cells that use inexpensive silicon substrates contain greater amounts of impurities and crystalline defects, which limit the internal collection depth of photo-generated carriers. Hence, silicon solar cells using inexpensive substrates generally have a carrier-collection junction (i.e., “emitter layer”) on the front surface, where most of the light is absorbed, in order to obtain a high collection efficiency of photogenerated carriers. For silicon solar cells, the bulk Si substrate is generally doped p-type and the emitter layer is generally a thin, heavily-doped, n+ layer that is formed in an “emitter diffusion” step by solid-state diffusion of phosphorus at elevated temperatures.
State-of-the-art back-contacted solar cells use an array of laser-drilled holes to effectively “wrap” the emitter layer from the front surface around to the back surface (hence, the name “Emitter Wrap Through”, or EWT). The laser-drilled holes are closely spaced about 1–2 mm apart, and typically have a diameter of about 50–150 microns (after chemical etching). The emitter diffusion step (i.e., doping with phosphorus) dopes both the exposed exterior front and back silicon surfaces n-type (except for any masked areas), as well as doping the exposed interior silicon surfaces n-type (i.e., n-doped) that are located inside of the hollow laser-drilled holes. The interior n-doped surfaces of the holes form an electrically conductive channel or pathway (i.e., “via”) that electrically connects the n-doped emitting front surface to the n-doped rear surface, thereby permitting negative ohmic contacts (i.e., current-collection grids) to be made on the rear surface. n-doping of the laser drilled holes also provides electrical insulation of the conductive via from the p-type bulk semiconductor. Efficient back-contacted solar cells using both photolithographically-defined and screen-printed metallizations have been demonstrated using laser-drilled holes diffused with phosphorus, i.e., in an EWT configuration, as disclosed in U.S. Pat. No. 5,468,652 to Gee, which is incorporated herein by reference. The back-contacted positive and negative grid lines are interdigitated and optimized to minimize electrical resistance and carrier recombination.
Disadvantages of an EWT design that uses laser-drilled holes to make conductive vias is the need for drilling a large number of holes (e.g., 600 holes/in2), and the added cost and manufacturing time of performing the laser-drilling step. Additionally, the conductivity of the laser-drilled vias may be limited by the achievable concentration of n-type dopant, and by the depth of the emitter diffusion layer realized on the interior surfaces of the laser-drilled holes. In other words, the performance of laser-drilled/diffused EWT cells is limited by the need to perform a single heavy diffusion to achieve sufficiently high conductivity in the laser-drilled vias.
Certain embodiments of the present invention use a gradient-driven migration process (e.g., thermomigration or electromigration) to create a conductive channel (i.e., via) in a single step, rather than a two-step process that laser-drills an array of holes followed by phosphorus doping to make it conductive. Simultaneous fabrication of an array of closely-spaced conductive vias using a gradient-driven migration process (as in the present invention) would reduce the cost and time needed to fabricate a back-contacted solar cell, since multiple vias can be fabricated simultaneously (rather than waiting for each hole to be individually laser-drilled, one-at-a-time). The equipment for performing thermomigration is less expensive than the equipment used for laser drilling. Also, the use of thermomigration may also produce vias with higher conductivity, as compared to laser drilled-holes.
Finally, a thermomigrated back-contacted cell can have a more optimally-doped front surface for higher conversion efficiency. In standard front-contacted cells, the front emitter must be doped heavily enough to achieve low contact resistance. Some photocurrent is lost in heavily doped emitters due to increased recombination losses in the emitter. The simplest process sequence for a back-contacted cell using an EWT geometry and laser-drilled holes uses a heavy emitter diffusion to minimize series resistance inside the holes and contact resistance to the grid. In a back-contact cell using thermomigration vias, the front surface n+ diffusion and the conductive via doping can be performed in separate steps—so each step can be separately optimized. Hence, a more optimally-doped front surface comprising a lower doping concentration can be used for thermomigrated cells, which results in increased photocurrent collection.
Another advantage of thermomigrated vias is lower series resistance. A thermomigrated conductive via provides lower series resistance compared to diffused holes in the EWT cell because it is a solid cylinder that is doped, while the EWT cell only has a thin surface n+ diffusion in the walls of the laser-drilled holes. If the thermomigration metal is left in place after the thermomigration, it should yield a much lower contact resistance, as well. Finally, if the thermomigrated via is a pattern of lines rather than holes, losses due to current flow in the emitter is reduced. Also, if the thermomigrated via pattern is a pattern of lines, the thermomigration metal on the rear surface may be left on the surface and used as the n-type grid.
Thermomigration (i.e., Soret effect) is a gradient-driven migration process that refers to the movement of a liquid solute (e.g., drop, droplet, wire) through a solid host in the presence of a temperature gradient (i.e., thermal gradient). The physics of the process involves more-rapid dissolution of the solid host at the front (hot) surface of the liquid drop, which causes a supersaturation of the solute at the rear (cold) surface of the liquid drop where it is deposited. In other words, liquid droplets (typically, metals) migrate inside a solid host in the direction up the thermal gradient (migrating from cold to hot) because atoms of the solid host dissolve into the liquid at the hot interface of the droplet, diffuse across the droplet, and deposit on the cold interface of the droplet. The resulting flux of dissolved solid host atoms from the hot to the cold side of the liquid droplet causes the droplet to migrate in the opposite direction, namely, towards the hot end of the host. For metallic droplets, the minimum temperature of the semiconductor substrate must be above the semiconductor/metal eutectic temperature for thermomigration to work.
Using the process of thermomigration, sheets, wires, or drops of a suitable metallic liquid can move through the bulk of a semiconductor material under the influence of a thermal gradient, leaving behind a trail comprising recrystallized solid host material doped with a solid solution of dopant material left behind by the passage of the liquid phase. Thermomigration can form conductive channels (vias) of heavily doped silicon if the liquid metal droplet contains (or is) a dopant having opposite polarity of the substrate. Thermomigration, also called temperature gradient or Thermal Gradient Zone Melting (TGZM), of fine droplets and planar zones has been shown to be a stable process when the dimensions of these zones are sufficient small. The TGZM process was first patented by Pfann in 1957. See also H. E. Cline and T. R. Anthony, Journal of Applied Physics, Vol. 47, No. 6, June 1976. Cline and Anthony have measured aluminum-rich liquid wires and dots/droplets migrating through n-type silicon at 1200 C. with an applied thermal gradient of about 50° C./cm at a velocity of about 1 mm/hour. In this experiment, the recrystallized silicon deposited behind the migrating aluminum-rich droplet was doped sufficiently high with a residual solid solution of aluminum (n-dopant) to convert the original n-type silicon matrix/host to p-type silicon inside of the recrystallized columnar/cylindrical droplet trail that was left behind. It is known that aluminum, gold, and germanium can be thermomigrated through silicon, germanium, or gallium arsenide.
Thermomigration has been used to fabricate p-n junctions in solar cells. See, for example, U.S. Pat. No. 3,936,319 to Anthony, et al.; U.S. Pat. No. 4,190,852 to Warner; and U.S. Pat. No. 4,173,496 to Chaing, et al. However, this technique has not been used to fabricate completely back-contacted solar cells.
In a gradient-driven transport process, such as thermomigration (or electromigration), a gradient in temperature (or electric field) determines the direction for transporting/migrating the solute material. In the fabrication of silicon solar cells, the direction of the thermal gradient is typically oriented across the thickness of the silicon substrate/wafer (i.e., in a direction perpendicular to the plane of the silicon substrate). Such a thermal gradient can be produced by heating one side of the substrate, rather than both sides (which would produce a uniform temperature).
In typical semiconductor production lines, the temperature of a silicon wafer sometimes needs to uniformly increased rapidly, and then held for a short period of time, e.g., 60 seconds, for annealing or diffusion treatments. This is often accomplished by using Rapid Thermal Processing (RTP) equipment, which typically consists of a double-sided bank of tungsten halogen quartz heating lamps that heat both sides of the wafer uniformly, so that there is essentially no temperature variation across the wafer or through its thickness. Unacceptably large temperature variations would create non-uniform annealing rates, diffusion rates, etc. and may result in poor reliability, reduced yield, etc. Hence, RTP equipment is carefully designed to heat the silicon wafers as uniformly as possible; and to minimize any temperature gradients across the thickness of a wafer.
Electromigration is another type of gradient-driven migration process, similar to thermomigration that can be used to move metal droplets through a semiconductor material. For more details, see U.S. Pat. No. 4,377,423 to Anthony, which is incorporated herein by reference. In this process, an electric field gradient (potential) provides the driving force for moving liquid metal droplets through the thickness of a semiconductor material, such as liquid aluminum drops through silicon.
What is needed, however, is an economical and reliable process that uses thermomigration (or some other type of gradient-driven solute transport technique, such as electromigration) to create an array of closely-spaced n++ conductive vias that electrically connect an n+ emitter layer on the front surface of a solar cell to ohmic contacts located on the cell's backside. Against this background, the present invention was developed.